Radio interference near FM radio towers.. Solution

SloRoam

SloRoam

Pre-Flight
Joined
Apr 4, 2019
Messages
98
Display Name
Display name:
SloRoam
Was talking with a guy on my local strip here who flies into Seattle quite a bit. It seems every time he went past a certain hill with quite a few FM radio transmitters, his com radios were lit up with static for a few minutes.

This was so bad he couldn't talk with tower or with ATC.

We started investigating likely culprits and decided to try an FM notch filter between his ELT and ELT antenna. It's a legacy ELT, so the notch filter, which filters out FM radio bands, completely solved the problem.

I was fairly surprised that that would be the issue, but it started to make sense when we realized that the ELT was likely amplifying the FM signal being picked up by the ELT antenna.

Hope this could help somebody else!
 
if I am between hangars, using comm #2 will activate my ELT.
Would something like this help my problem or is it too remote from what you are talking about?
 
Cougar Mountain, near Issaquah.

I may remove my ELT and see if the interference still happens....

Ron Wanttaja
 
Read a thing once, I think it was here on POA. Guy was having a problem like that. A&P got a wire cutter and snipped a little off of the ELT antennae. It worked. The length has something to do with the frequencies it responds to. Don’t remember if he checked to see if the ELT still broadcast on 121.5
 
luvflyin said:
Read a thing once, I think it was here on POA. Guy was having a problem like that. A&P got a wire cutter and snipped a little off of the ELT antennae. It worked. The length has something to do with the frequencies it responds to. Don’t remember if he checked to see if the ELT still broadcast on 121.5
Click to expand...
A perfect 1/4 wavelength antenna would be 24 & 9/32 inches long @ 121.5mhz. I suppose someone that was familiar with antenna theory could've measured the antenna and clipped a little off to change the characteristic impedance slightly. It wouldn't cause any change to the transmit frequency, just the efficiency of the antenna. To know for sure how it was changed would take an antenna analyzer.

Anyone who flies in and out of Renton, near downtown Seattle, is familiar with the antenna farm just to the east. Everyone complains about RFI around that hill.
 
Last edited:
wilkersk said:
A perfect 1/4 wavelength antenna would be 24 & 9/32 inches long @ 121.5mhz. I suppose someone that was familiar with antenna theory could've measured the antenna and clipped a little off to change the characteristic impedance slightly. It wouldn't cause any change to the transmit frequency, just the efficiency of the antenna. To know for sure how it was changed would take an antenna analyzer.

Anyone who flies in and out of Renton, near downtown Seattle, is familiar with the antenna farm just to the east. Everyone complains about RFI around that hill.
Click to expand...

The number I get is 23 1/8" for 121.5 MHz on the calculators I try...what's different about your number?

The number I get for FM is about 26" at the high end. So, yeah, trimming some length would make a large efficiency difference for FM with a smaller difference at the ELT freq.
 
arnoha said:
The number I get is 23 1/8" for 121.5 MHz on the calculators I try...what's different about your number?

The number I get for FM is about 26" at the high end. So, yeah, trimming some length would make a large efficiency difference for FM with a smaller difference at the ELT freq.
Click to expand...

I did not calculate velocity factor. You probably used 96% efficiency.
 
Last edited:
You have to take into account the velocity factor of what you're building the antenna out of. It will be close to, but by definition less than one.
 
wanttaja said:
Cougar Mountain, near Issaquah.

I may remove my ELT and see if the interference still happens....

Ron Wanttaja
Click to expand...
If you have an ACK E-04 installed, there is a service bulletin requiring a static filter between the ELT and the antenna. The newer versions of the E-04 come with the required filter.
 
I know the exact place you are talking about. Was really bad in my buddies Mooney, this annual he removed antenna cleaned up bonding, re-installed antenna and replaced coax with the new good stuff. Eliminated the problem completely at the Issaquah towers area.
 
You must log in or register to reply here.
Back
Top